The present invention relates to an apparatus and method for the recovery or winning of a metal from a waste metal material, and more particularly, to a process for the electrowinning of a metal, e.g. nickel, from a metal hydroxide press cake.
In the conventional electrowinning of metals, the metal to be electrowon is first dissolved by chemical means in the electrolytic cell containing an insoluble anode. Conventional electrowinning processes employ cells containing planar or grid-type anodes and planar cathodes. The metal ions in the electrolyte are discharged at the cathode surface by passage of a direct electric current, to form a deposit of relatively pure metal, while negative ions, such as hydroxyl or sulfate ions, are discharged at the anode and then reform with the release of molecular oxygen bubbles. The electrolyte usually consists of an aqueous solution of one or more salts of the metal which is in solution so as to promote electrodeposition of the metal on the cathode in such form and purity as is desired.
Conventional electrowinning cells usually require large cathode surface areas. U.S. Pat. No. 4,129,494 to Norman discloses the use of about 50 or more cathode sheets or plates with a total active surface area of greater than 1000 square feet. This large surface area is exceedingly costly. Not only do the large number of cathodes add to the capital costs, but the often desirable high current densities lead to high operating costs and difficult design problems in obtaining adequate current. Current density is defined as the ratio of current in amperes to the area of cathode. Thus, the larger the cathode surface area is, the greater the current must be.
Additionally, the requirement of first dissolving the metal in an acidic solution is costly and time consuming since the resulting solution oftentimes must be treated to remove the excess acid. Then, once the solution undergoes electrowinning, some mechanism is required to neutralize or recover the regenerated acid.
These and other limitations and disadvantages of the prior art are overcome by the present invention which eliminates the necessity for dissolving the metal to be electrowon, and the prior art large surface area requirements.